I was lucky enough to attend and participate in one of Boal's workshops in the mid-1990's.

Generally speaking, I think that there are two parts in an answer to your question...

You are very astute to assume that different ethnic or financial backgrounds would being away different bits of learning from his workshops. however, that can be true given any set of difference, not limited to the two you've proposed (such as age, gender, etc.). Overall, from my experrience not only participating but observing the other members, I would only be comfortable in saying that everyone was able to contribute differently based upon their personal history, and similarly learned different things based upon that same history.

As an example, being male, I was presented with many issues surrounding poor single mothers that I had not really considered. The poverty did not strike me as much (as I came from poverty), but its effects upon a woman was certainly information I hadn't really considered.

Similarly, given the "hidden" and public nature of many of his performances, I would also say that his audiences will draw upon their own histories and bring into their lives those parts of his show that seem to resonate with them the most, or perhaps that cause the most cognitive dissonance.